This invention generally relates to a layered substrate structure for subsequent epitaxial growth of a III-V nitride semiconductor. The III-V nitride semiconductor is a III-V compound semiconductor which contains nitrogen as an element, and is written as InGaAlBNAsP alloy (InxGayAlzBwNαAsβPγ alloy, where x+y+z+w=1, α+β+γ=1, and 0<=x,y,w,α,β,γ<=1). Such wide band gap III-V nitrides are widely used for visible light emitting diodes (LEDs) in many applications. The excitation of fluorescent material using blue or ultraviolet LEDs could enable the emission of white light, which could replace current light bulbs with bulbs with longer lifetimes. A high-density optical disk system is also a promising application of blue or ultraviolet semiconductor lasers using III-V nitrides. At present, test systems of high density optical disks are available using III-V nitride ultraviolet lasers.
A conventional substrate for the formation of III-V nitrides is sapphire on which III-V nitrides are grown by using metal organic chemical vapor deposition (MOCVD). Improvements in crystal quality have been seen by inserting a buffer layer grown at a low temperature between the substrate and a layer grown at a high temperature. Another alternative substrate is silicon carbide, however, SiC is still more expensive than sapphire even though SiC contains more crystal defects.
A prior art substrate and subsequent epitaxial growth process using III-V nitrides by MOCVD is explained with FIGS. 1A–1D. FIG. 1A shows an illustration of a prior art MOCVD system for epitaxial growth of III-V nitrides. Trimethyl-gallium(TMGa), trimethyl-aluminum(TMAl), trimethyl-indium(TMIn) and ammonia (NH3) are used as source gases 112. Hydrogen is used as a carrier gas 113. A sapphire substrate 110 is placed on a susceptor 116 which is heated by using a radio frequency (RF) inductive heating system 118. Susceptor 116 is typically composed of graphite. Substrate 110 and susceptor 116 are both encased in a water-cooled reactor tube 114. RF inductive heating system 118 comprises an induction coil.
Referring to FIG. 1B, III-V nitride layers 122 are grown on a sapphire substrate 110 as seen in the cross sectional process flow. Typically, sapphire substrate 110 is 300 μm thick. Nitride layers 122 may include a GaN layer, an InGaN layer, and/or an AlGaN layer. First, a 50 nm thick AlN buffer layer is grown on sapphire substrate 110 at about 500° C. Then, epitaxial III-V nitride layers including a pn junction of InAlGaN alloys are grown on the AlN buffer layer. Typically, the epitaxial layers contain AlGaN cladding layers and InGaN quantum well active layers. After the growth of the III-V nitride layers, the resulting wafer is cooled down to room temperature and unloaded from reactor tube 114. At this point, the wafer exhibits bowing in a convex manner as illustrated in FIG. 1B. This bowing is caused by (1) the thermal mismatch between sapphire substrate 110 and nitride layers 122 and (2) the nitride layers 122 being affixed to sapphire substrate 110 at an elevated temperature.
Referring to FIG. 1C, the radius of the bowing can be calculated by using a simple model proposed by Olsen et. al., in which balance of forces and the momentum caused by the thermal mismatch are considered. FIG. 1C shows the calculated bowing for various GaN thicknesses based on the Olsen model. In this calculation, a simple two-layer model is used. The bowing becomes significant with the increase of the thickness of nitride layer 122 and can cause a significant decrease in yield.
Referring to FIG. 1D, the wafer is patterned through a photolithography process, followed by metallization, dielectric film deposition, and so on. With such a bowed wafer, fine patterning in the peripheral area is difficult and results in low yields for devices fabricated across the entirety of the wafer. FIG. 1D shows in cross-section an example of a light emitting diode (LED) fabricated in this manner from InGaN/AlGaN heterostructures. A pn junction structure is grown on sapphire substrate 110. Since sapphire is an insulating material, a p-type layer 136 and an active layer 134 are selectively etched, and a p-electrode 140 and a n-electrode 138 are formed. Typically, p-type layer 136 is composed of GaN or AlGaN. Typically, active layer 134 is composed of InGaN.
The epitaxial layer thickness 139 is limited to around a few microns in order to reduce the wafer bowing due to the thermal mismatch. Since the current flows through a very thin n-type layer 132, a series resistance 130 of the LED is high, resulting in high voltage operation. Typically, n-type layer 132 is composed of AlGaN or GaN. Even a small device chip may bow in a convex manner so that precise die bonding may be very difficult.
It is therefore highly desirable to provide a wafer which remains flat after III-V nitride growth and which is suitable for subsequent high-yield processing of high performance devices.